Perforated trays with downcomers are one of the most commonly used internals in industrial columns or towers for contacting a gas and a liquid. In such columns, liquid flows across the trays and down through the downcomers from tray to tray, while gas permeates upwardly through the trays foaming liquid thereon.
Many of these columns, which may be used for distillation, absorption and stripping operations, involve having solids present in particulate form, see for example, xe2x80x9cSubdue Solids in Towerxe2x80x9d, A. W. Stoley et al, Distillation and Other Industrial Separations, pages 95-104, dated January 1995, published in Chemical Engineering Progress. Stoley et al, center column, lines 25-38, page 99, states that xe2x80x9ca major problem with downcomers is the creation of dead spots near the tower wall (opposite the downcomer outlet), near outlet wiers, and along the tower shell at the bottom of the downcomerxe2x80x9d.
Various proposals have been made, see, for example, U.S. Pat. No. 2,646,977, dated Jul. 28, 1953, U.S. Pat. No. 4,749,528, dated Jun. 7, 1988, J. T. Lavin, U.S. Pat. No. 5,895,608, dated Apr. 20, 1999, A. T. Lee et al and U.S. Pat. No. 6,003,847, dated Dec. 21, 1999, A. T. Lee et al, to reduce the tendency of liquid to dwell at the periphery of the tray adjacent the column wall so that distillation is equally effective at the peripheral and central areas.
While these proposals have achieved various degrees of success in avoiding stagnant zones in liquid adjacent the column wall, no one has appreciated that
i) narrowing of the downcomer passage sides adjacent to the column wall, all of the way down the downcomer, will always cause stagnant zones in the liquid at the extremities, and
ii) even with these narrow extremities eliminated to some extent, there is still a problem in that liquid entering the downcomer and flowing down the downcomer at the narrow extremities will flow slower at the entry causing back-up resulting in sedimentation of particulate solids to occur at the entry.
There is a need for a gas/liquid contacting apparatus wherein the formation of stagnant zones of liquid due to narrowing extremities of downcomers is substantially reduced, and where sedimentation of particulate solids due to back-up of downcomer liquid at these extremities is substantially reduced.
According to the present invention, there is provided a gas-liquid contacting apparatus, comprising:
a) an upwardly extending casing,
b) a series of perforated trays partitioning the casing at different levels and having generally crescent shaped, liquid drainage openings adjacent the casing, for, in operation, causing liquid descending in the casing to flow across the trays and be frothed thereon by gas, to be contacted therewith, with is ascending in the casing and permeating the perforations in the trays, and
c) for each drainage opening,
i) a liquid overflow weir, adjacent thereto and bowing outwardly to follow the contour of the entry to that drainage opening, and for maintaining a level of liquid flowing on the tray being drained by that drainage opening, and
ii) an inclined,xe2x80x94overflow liquid conveying, downcomer chute; the downcomer chute comprising a base and outwardly extending side walls, the upper ends of which generally conform with the contour of the bow of the overflow wier after which the side walls gradually converge inwardly on the base, from upper ends, to form a chute leading to lower ends, at which conveyed liquid is deposited from the chute on to a portion to the tray therebelow.
In some embodiments of the present invention, the. overflow weirs are formed of a plurality of substantially flat strips which are joined at oblique angles to form the bow shape of the overflow weirs.
In other embodiments of the present invention, each downcomer chute comprises a plurality of lengthwise extending substantially flat panels, joined along their lengthwise extending edges to provide the base and the side walls.
In other embodiments of the present invention the drainage openings are crescent shaped, and the weirs and downcomer chutes are curved to blend smoothly with the crescent shapes of the drainage openings.
In yet other embodiments of the present invention, each drainage opening is one of two similar side drainage openings provided at opposite sides of each perforated tray, each overflow weir is one of two similar overflow weirs for the side drainage openings, and each downcomer chute is one of two similar downcomer chutes for the weirs, and between each pair of perforated tray with two opposed drainage openings, a further perforated tray is provided comprising two segments, each to receive liquid from one of the downcomer chutes, the perforated tray segments providing a central, downcomer opening between them, the central downcomer openings being flared at the sides, and for each side of the central downcomer openings, there is provided
i) a weir conforming to the contour of that side, and
ii) a downcomer chute comprising a base and two side walls conforming to the contours of the weir, the side walls gradually converging inwardly on the base, to form a chute therewith for depositing liquid on a perforated tray therebelow with drainage openings on opposite sides.
In yet other embodiments of the present invention each drainage opening is one of two, similar, side drainage openings provided at opposite sides of each perforated tray, and
a) each overflow weir is one of two similar overflow weirs for the side drainage openings, and each downcomer chute is one of two similar downcomer chutes for the weirs, and a central drainage opening is provided in each perforated plate, the central downcomer openings being flared at the sides, and for each side of the central downcomer opening, there is provided
i) a weir conforming to the contour of that side,
ii) a downcomer chute comprising a base and two side walls the side walls conforming to the contours of the weirs, and gradually converging inwardly on the base, to form a chute therewith, and
b) for each perforated tray with side and central drainage openings, there is provided a further perforated tray therebelow, each further perforated tray having two longitudinally extending, intermediate, drainage openings, each extending along a path between a side drainage opening and a central drainage opening in the perforated tray there above, for each side of an intermediate drainage opening there is provided,
iii) a weir conforming to the contour of that side, and
iv) for each weir a downcomer chute conforming to the contour of that weir and gradually converging inwardly therefrom to form a chute therewith for depositing liquid on a perforated tray therebelow.